[MARKO-3] - <assign> and <var> tag migration

[t-l9]

Added <var> and <assign> migration transforms so users can still use the deprecated syntax, but allowing the reference to not be a breaking change in Marko V4.

Description

Both the <assign> and <var> tag have their attributes looped through to get transformed from a type of HtmlAttribute to a Scriptlet.

In the assign tag, one of two cases happen. Either the attribute name gets assigned a value, or the attribute name's value remains undefined. During this assignment, the new Scriptlet that is created is inserted as a sibling before the original elNode. After, the original elNode is detached.

The var tag has similar functionality to the assign tag. However, there are two differences. While looping through the attributes of the elNode, the attribute names are actually being defined as a variable and assigned rather than only being referenced. Again, during this assignment, the new Scriptlet that is created is inserted as a sibling before the original elNode. Before the detachment of the original elNode, it's children are looped over and inserted as a sibling before the original elNode to not lose the children that are referencing the declared variable.

Allows users to reference deprecated code without it breaking on compilation because the way the user wrote to code is transformed into how the code should be written.

Checklist:

• I have read the CONTRIBUTING document and have signed (or will sign) the CLA.
• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

[jsf-clabot]

All committers have signed the CLA.

[DylanPiercey]

@timlauter this looks good 😄! Would you mind updating the two failing snapshot tests to get the CI passing?

[codecov]

Codecov Report

Merging #1158 into master will decrease coverage by 0.03%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #1158      +/-   ##
==========================================
- Coverage   90.52%   90.48%   -0.04%     
==========================================
  Files         311      311              
  Lines       11803    11808       +5     
==========================================
  Hits        10685    10685              
- Misses       1118     1123       +5

Impacted Files	Coverage Δ
src/taglibs/core/assign-tag.js	85.71% <90%> (+2.38%)	⬆️
src/taglibs/core/var-tag.js	95.83% <94.44%> (-4.17%)	⬇️
src/compiler/ast/VariableDeclarator.js	74.07% <0%> (-11.12%)	⬇️
src/compiler/ast/UpdateExpression.js	87.5% <0%> (-2.5%)	⬇️

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[coveralls]

Coverage decreased (-0.04%) to 90.489% when pulling 623ab8e on assign-invoke-var-tag-migration into 4578da9 on master.